How can I alter this computed column in SQL Server 2008?

问题

I have a computed column created with the following line:

alter table tbPedidos 
add restricoes as (cast(case when restricaoLicenca = 1 or restricaoLote = 1 then 1 else 0 end as bit))

But, now I need to change this column for something like:

alter table tbPedidos 
alter column restricoes as (cast(case when restricaoLicenca = 1 or restricaoLote = 1 or restricaoValor = 1 then 1 else 0 end as bit))

But it's not working. I'm trying to input another condition to the case statement, but it's not working.

Thanks a lot!

回答1:

If you're trying to change an existing column, you can't use ADD. Instead, try this:

alter table tbPedidos alter column restricoes as (cast(case when restricaoLicenca = 1 or restricaoLote = 1 or restricaoValor = 1 then 1 else 0 end as bit))

EDIT: The above is incorrect. When altering a computed column the only thing you can do is drop it and re-add it.

回答2:

Something like this:

ALTER TABLE dbo.MyTable
DROP COLUMN OldComputedColumn

ALTER TABLE dbo.MyTable
ADD OldComputedColumn AS OtherColumn + 10

Source

回答3:

This is one of those situations where it can be easier and faster to just use the diagram feature of SQL Server Management Studio.

1. Create a new diagram, add your table, and choose to show the formula column in the diagram's table view.
2. Change the columns formula to an empty string ('') or something equally innocuous (probably such that you don't change the column's datatype).
3. Save the diagram (which should save the table).
4. Alter your function.
5. Put the function back in the formula for that column.
6. Save once again.

Doing it this way in SSMS will retain the ordering of the columns in your table, which a simple drop...add will not guarantee. This may be important to some.

回答4:

Another thing that might be helpful to someone is how to modify a function that's a calculated column in a table (Following query is for SQL):

ALTER <table>
DROP COLUMN <column>

ALTER FUNCTION <function>
(
<parameters>
)
RETURNS <type>
BEGIN
...
END

ALTER <table>
ADD <column> as dbo.<function>(parameters)

Notes:

1. Parameters can be other columns from the table

2. You may not be able to run all these queries at once, I had trouble with this. Run them one at a time

3. SQL automatically populates calculated columns, so dropping and adding won't affect data (I was unaware of this)

回答5:

As Michael Todd correctly states in his answer

When altering a computed column the only thing you can do is drop it and re-add it.

Just had to do this myself, although I wanted to preserve the existing data (as Management Studio does when you perform this task through the designer).

My solution was to store the data in a temporary table then update the table with the stored values after I'd dropped and recreated the calculated column.

SELECT IDKey, Value
INTO #Temp
FROM MyTable

ALTER TABLE MyTable
DROP COLUMN Value

ALTER TABLE MyTable
ADD Value nvarchar(max) NULL

UPDATE MyTable
SET Value = #Temp.Value
FROM
MyTable
INNER JOIN #Temp ON #Temp.IDKey= MyTable.IDkey

来源：https://stackoverflow.com/questions/2372655/how-can-i-alter-this-computed-column-in-sql-server-2008